1. Field of the Invention
The present invention relates to printing, specifically to printing on moving substrates, and more specifically to printing on moving flexible sheets and films, particularly those of the type used in product packaging.
2. Background of the Invention
The printing of moving substrates, particularly flexible thermoplastic substrates such as are used in packaging, continues to be an area of industrial interest and research. The printing of such substrates presents several issues including short-term and long-term adhesion, opacity, resistance of the substrate to blocking, resistance of the substrate surface and the printed image to abuse, scuffing of the printed image, and the like.
Among the various substrates that undergo printing, flexible sheets and films (hereinafter collectively referred to as "films") used for packaging applications present special problems resulting from the variety of end-use requirements which they are expected to meet. Specifically, packaging made from or incorporating such films must be capable of adequately protecting the product(s) enclosed therein. Depending on the particular product, characteristics required of such films include, inter alia, providing a barrier to one or more gases (e.g., oxygen), providing high transmission of one or more gases (e.g., carbon dioxide and/or oxygen), odor retention or transmission, resistance to abuse, easy sealability, provision of a strong seal(s), ability to withstand elevated temperatures and/or pressures, and the like. The particular combination of requirements of course depends on the particular end use application.
Despite the variety of end use applications to which packaging films are put, they nevertheless must be able to display words and pictorial images in a manner that informs consumers and entices them to purchase the product packaged therein. To do so, the printed words and/or images must be adhered securely to a layer of the film, normally the outermost layer. Depending on the particular film structure mandated by the particular end use application, whole new sets of problems are presented to the skilled artisan.
Because of their desirable sealing characteristics, films with polyolefin sealing layers are used for a variety of end use applications. Of the polyolefins, ethylene-based layers are most widely used. However, many inks used in the printing of films are nitrocellulose/polyamide or nitrocellulose/polyurethane-based formulations. Because of chemical incompatibility between polyolefin-containing layers and such inks, printed images often do not adhere well to films with polyolefin-containing outer layers. This lack of adherence manifests itself sometimes immediately, e.g., the ink smears while the film is being further processed, and sometimes after the passage of time, e.g., the ink lifts off the film onto the fingers of the end-use consumer.
Many solutions have been proposed to overcome this problem. One solution is to print the image(s) on another, more compatible layer of the film and then to laminate the polyolefin seal layer over the printed portion of the film. This method often is called trap printing. However, it involves a separate lamination step which involves the added costs and difficulties of an additional manufacturing step. Additionally, trap printing becomes impracticable where the film in question is oriented and its end use involves heat shrinking. This is due to the tendency of the films to shrink at different rates, thus resulting in a distorted image.
Another more common solution is to chemically or physically prime the outer surface of the seal layer so as to make it more receptive to ink(s) being applied thereto. Physical priming involves roughening the outer, to-be-printed surface by, for example, an oxidizing treatment such as corona discharge or flame. Although physical priming solves the ink adhesion problem, the roughened surface can behave differently for other purposes such as, for example, machinability. Loss of machinability can result in lower productivity and throughput. Additionally, surface treatments can crosslink polymer chains, especially those in the surface layer, which can decrease the sealability of the film. Further, such surface treatments tend to degrade over time, especially where surface active agents (e.g., slip agents, antifog agents, etc.), thus limiting the shelf-life of the film.
Chemical priming involves the application of a layer of a material that acts to compatibilize the layer in question and the ink(s). Typical primers include ethylene/vinyl acetate copolymer (hereinafter "EVA") and/or ethylene/acrylic acid copolymer (hereinafter "EM"). However, application of a separate primer layer not only adds time and expense to the printing process, it also lessens the numbers of colors that can be applied to a given film. Specifically, a press operator normally must substitute primer for the pigmented ink normally supplied by the first print station. Thus, printing flexibility can be lost and tedious press set up and cleaning can be incurred due to the use of a primer. Further, because primers typically are clear, achieving proper registration is very difficult
Where priming of the film surface is undertaken, regardless of whether that priming is chemical or physical in nature, the resulting benefits can be lost over time due to migration of slip agent from the interior of the film to the outer surfaces thereof. Often, a slip agent such as, for example, an amide-based wax commonly is added to increase the machinability of a film by reducing its coefficient of friction. However, not surprisingly, such waxes also reduce the ability of films to adhere to ink(s) printed thereon.
Additionally, for many end use applications, one or more antifogging agents are added to the blend from which surface layers are made. Antifogging agents decrease the surface tension of the surface layers; however, in the process, they also can interfere with the ability of ink to adhere those same surface layers. The tendency of antifogging agents to migrate away from a surface where they are included also is well known.
The manufacture of certain types of films can involve the use of lubricants (e.g., silicone) on the surface of the film to aid in machinability during processing. Such lubricants also can interfere with the ability of ink to adhere to the surface layer(s).
Thus, a need remains for a method of printing films that include a polyolefin-based seal layer in which that layer need not be primed. Preferably, such a method can account for the decrease in printability associated with the presence of slip and antifogging agents.